In 1859, Charles Darwin published On the Origin of Species (Alfred Russel Wallace had reached the same idea on his own), explaining how populations change by natural selection. It joins what you know about adaptations and Alleles: Dominant and Recessive.
Key idea
Natural selection needs four things: (1) variation: individuals differ; (2) inheritance: some of those differences are passed on in genes; (3) more young than can survive, so there is competition; (4) selection: individuals with helpful traits survive and reproduce more, so those traits become more common over generations.
Why it works
Nothing in the process needs a beetle to try to change. Each beetle is born with its color. Light beetles on dark soil are spotted and eaten more often, so fewer of them survive to have young. Dark beetles leave more offspring, and the offspring inherit dark color. Repeat over many generations and the population becomes mostly dark. The individuals did not change; the mix of the population did.
Worked example: Spot the four parts
A field mouse population has fur from tan to dark brown. Fur color is inherited. Owls hunt at night over pale sand, and each year many more mice are born than survive. What happens?
- Variation: tan to dark brown fur.
- Inheritance: fur color is passed on.
- Too many young: not all survive.
- Selection: owls spot dark mice on pale sand more easily, so tan mice survive and reproduce more.
Answer: Over generations, tan fur becomes more common.
Common mistake
Students think individuals change their traits because they need to, like a giraffe stretching its neck and passing on a longer one. Stretching is an acquired trait and is not inherited. Individuals do not evolve; populations do, as inherited traits that help survival become more common.